There are many devices used to indicate if a round is in a chamber of a handgun. For example, U.S. Pat. Nos. 6,857,213, 6,785,994, 6,622,411, 6,493,977, 6,161,322, 6,256,915, 5,926,987, 5,826,360, 3,997,994 and 6,094,850 have various designs for round-in-the-chamber indicators, wherein a lever springs up (typically from the slide or upper portion of the handgun) when a round is in the chamber.
The XD model handgun of Springfield Armory has two indicators—a striker status (cocked) indicator and a loaded chamber indicator. The loaded chamber indicator is a small button, just above the breech on top of the gun's slide, which pops up (by means of mechanical action) when a round is in the chamber. The button does not interfere with the shooter's line of sight, but is high enough to be seen easily, or felt by hand (e.g., for use in the dark). The striker status indicator works much the same way (spring-loaded mechanical action), but is located on the rear face of the slide, so the shooter can instantly tell whether or not the gun is cocked. The striker status indicator pops rearward out of the rear face of the slide when the gun is cocked and is flush with the rear face of the slide when the gun is not cocked.